Add pause_eventloop for do-block syntax

[IanButterworth]

A do-block syntax like this is helpful for running functions where you explicitly don't want the eventloop running.

pause_eventloop() do
    foo()
end

[codecov]

Codecov Report

Merging #613 (edc5806) into master (c23b075) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #613      +/-   ##
==========================================
+ Coverage   57.44%   57.55%   +0.11%     
==========================================
  Files          32       32              
  Lines        2700     2707       +7     
==========================================
+ Hits         1551     1558       +7     
  Misses       1149     1149              

Impacted Files	Coverage Δ
src/Gtk.jl	91.83% <100.00%> (+1.36%)	⬆️

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